Exhaust gas turbochargers are known to be used for increasing power of an internal combustion engine (combustion engine). An exhaust gas turbocharger can include a compressor which feeds air to the combustion chamber of the internal combustion engine for the combustion process, and an exhaust gas turbine in the exhaust gas tract of the internal combustion engine. With the charging of the internal combustion engine, the air and fuel volume in the cylinders is increased, and a noticeable power increase is produced for the internal combustion engine is produced as a result. The exhaust gas turbocharger is assembled from a rotor, which comprises a compressor impeller and a turbine wheel and also the shaft bearing, the flow-guiding casing sections (compressor casing, turbine casing), and the bearing housing.
If the internal combustion engine is operated under full load, and the exhaust gas turbine of the exhaust gas turbocharger is correspondingly exposed to a large exhaust gas flow, high circumferential speeds at the rotor blade tips of the turbine wheel and of the compressor impeller are reached. The maximum permissible rotor speed of a turbocharger is a function of the wheel size, the geometry and the strength values of the materials which are used. In general, the rotating components are subjected to high centrifugal force loads and therefore to high material stresses. Defects in the material microstructure can possibly lead to bursting of the compressor impeller or turbine wheel with unpredictable consequences for the adjacent casings.
The initial failure image of a compressor impeller can be described by a blade fracture or a multipiece hub burst. In the case of blade bursts, the blades fail in the root region of the compressor, wherein the impeller hub remains intact. In the case of multipiece hub bursts, the hub region can break into two to four fragments, for example. A significant case of compressor bursting is the 3-piece hub fracture with three fragments of approximately the same size (3×120° sectors). The burst protection concept (containment concept) of an exhaust gas turbocharger is designed to the effect that all the fragments, for the case of a multipiece hub burst, are retained within the outer casing shell at a prespecified burst speed. Thus, in the construction of the exhaust gas turbocharger, consideration is given to the fact that the kinetic energy of the compressor is already dissipated in the inner casing sections which are close to the rotor as a result of plastic deformation, and consequently the remaining kinetic energy of the radially outwardly thrown fragments is not sufficient to penetrate the outer casing shell or to cause the outer casing connections (for example bolts) to fail.
Different measures for the reduction of load of the casing connection in the case of a bursting compressor impeller are known.
According to WO 02/090722, a design break point is provided in the casing insert wall, which radially outwardly delimits the flow passage through the rotor blades of the compressor impeller, in order to prevent the axial spinning away of casing pieces or of components which are fastened on the compressor casing in the event of a compressor burst.
In EP 1-586-745, by means of a support flange and a sufficiently large distance of the support flange from the casing insert wall, a direct axial impulse transfer of flying-away compressor impeller pieces onto the air inlet casing taking place is prevented in the case of a bursting compressor impeller, and so the load of the upper connections between the casing sections is reduced and the breaking up of the connection and the emergence of fragments are prevented.
In a further variant according to GB 2-414-769, the axial load of the casing insert wall in the case of hub bursts is adequately absorbed by means of the long necked-down bolts, and the bolted flange connection between the compressor casing and the bearing housing is sufficiently unloaded.
In the variant according to DE 10-2004-028-133, such a necked-down bolt is provided with an additional precision fit between the bolt, the casing insert wall and the compressor casing. As a result of the precision fit, the circumferential forces which occur during bursting are absorbed, and rotation of the insert wall in relation to the compressor casing is avoided.
In DE 10-2005-039-820, the casing insert wall is supplemented with a retaining device in order to consequently trap or to jam the axially forwardly accelerated fragments of the compressor impeller and also of the casing insert wall.
The variants which are described above in most cases involve large constructional volumes for realizing the features which are described therein. Furthermore, in some variants, long necked-down, precision-fit bolts are used, which makes higher demands on the accuracy of casing manufacture, on the production costs and on the structural dimensions of the turbocharger. In DE 10-2005-039-820, DE 10-2004-028-133 and GB 2-414-769, the axially acting bursting forces are first absorbed by means of the necked-down precision-fit bolts and only then directed via the casing wall into the upper, shorter bolted connections between the compressor casing and bearing housing. These aforementioned bolted connections are the points of a compressor-side bursting concept which are to be protected.